1. Field of the Invention
The present invention relates to a method and system for compressing motion image information data by means of predictive coding with a high compression ratio and with high image quality.
2. Description of the Related Art
In image signal transmission, an image signal is first converted to another signal and then coded by assigning proper codes selected according to the statistical feature of the converted signal. When the image has high redundancy within a frame as is the case with an image including a regular pattern or a plain image, there is a high correlation between adjacent pixels, and thus it is possible to predict, to a certain degree, a pixel value to be coded next from a pixel value which has already been coded. Therefore, if components which could not be predicted are extracted, and the coding is performed only for these extracted components, it becomes possible to compress information with a large compression ratio. This technique is called the predictive coding.
In the case of a motion image in a video telephone or the like, images of adjacent frames are very similar to each other, and thus the temporal change is limited. Such temporal redundancy can be removed by means of interframe predictive coding on the basis of interframe prediction. In this case, a block code is generally employed in which one codeword is assigned to one symbol, and one frame is divided into a plurality of pixel blocks so that the luminance difference within each block becomes smaller than within the entire frame. The smaller luminance difference within each block allows compression of information. This technique is called block coding.
Entropy coding is known in the art as a coding technique which achieves data compression by assigning a high efficiency code to the converted signal. Huffman coding is known as a method for generating a high efficiency code. A representative example of the high efficiency code is an arithmetic code in which a probability numeric line is divided into segments in accordance with the occurrence probability of a symbol system, and a binary decimal number indicating a location in a segment is employed as a code for the symbol system. Codewords are generated one by one by performing the arithmetic process described above.
A 3-step block coding system is known in the art as a technique for coding an image signal in an efficient manner. This technique consists of 3 steps: sampling; conversion; and quantification. In this technique, in order to retain a two-dimensional resolution and high-frequency components of a given image signal, it is generally required to perform sampling at a frequency twice the highest frequency component.
However, in the conventional image signal compression technique, because of the complexity of the block coding process, it is difficult to achieve both a high compression ratio and a high speed in the predictive compression coding of image data or audio data.
When differential information is compressed in the motion image compression process, that is, when successive values A1 and A2 are predicted to be similar to each other and when the value A1 is known before the value A2 occurs, the difference A2−A1 is regarded as having a value near 0, and compression is performed using the conventional Huffman code or arithmetic code. However, if A1 and A2 can each take one of values 0, . . . , n, the difference A2−A1 can have one of 2n+1 values. As a result, it is required to prepare 2n+1 Huffman codewords. However, A2 can actually take one of n values, and thus n codes are not used from the local point of view. This means that the code includes a large amount of redundancy. Furthermore, when a difference between frames is great, significant degradation in image quality occurs, and a high-quality image cannot be obtained.
In view of the above, it is an object of the present invention to provide a method and system for compressing data such as image information or audio information by means of predictive coding with a high compression ratio while maintaining high image quality.